diff --git a/src/math/big/int.go b/src/math/big/int.go
index 63a750cb96..000eab50b7 100644
--- a/src/math/big/int.go
+++ b/src/math/big/int.go
@@ -390,15 +390,20 @@ func (x *Int) IsUint64() bool {
 // ``0b'' or ``0B'' prefix selects base 2. Otherwise the selected base is 10.
 //
 func (z *Int) SetString(s string, base int) (*Int, bool) {
-	r := strings.NewReader(s)
+	return z.setFromScanner(strings.NewReader(s), base)
+}
+
+// setFromScanner implements SetString given an io.BytesScanner.
+// For documentation see comments of SetString.
+func (z *Int) setFromScanner(r io.ByteScanner, base int) (*Int, bool) {
 	if _, _, err := z.scan(r, base); err != nil {
 		return nil, false
 	}
-	// entire string must have been consumed
+	// entire content must have been consumed
 	if _, err := r.ReadByte(); err != io.EOF {
 		return nil, false
 	}
-	return z, true // err == io.EOF => scan consumed all of s
+	return z, true // err == io.EOF => scan consumed all content of r
 }
 
 // SetBytes interprets buf as the bytes of a big-endian unsigned
diff --git a/src/math/big/intmarsh.go b/src/math/big/intmarsh.go
index ee1e4143ed..c1422e2710 100644
--- a/src/math/big/intmarsh.go
+++ b/src/math/big/intmarsh.go
@@ -6,7 +6,10 @@
 
 package big
 
-import "fmt"
+import (
+	"bytes"
+	"fmt"
+)
 
 // Gob codec version. Permits backward-compatible changes to the encoding.
 const intGobVersion byte = 1
@@ -52,8 +55,7 @@ func (x *Int) MarshalText() (text []byte, err error) {
 
 // UnmarshalText implements the encoding.TextUnmarshaler interface.
 func (z *Int) UnmarshalText(text []byte) error {
-	// TODO(gri): get rid of the []byte/string conversion
-	if _, ok := z.SetString(string(text), 0); !ok {
+	if _, ok := z.setFromScanner(bytes.NewReader(text), 0); !ok {
 		return fmt.Errorf("math/big: cannot unmarshal %q into a *big.Int", text)
 	}
 	return nil
diff --git a/src/math/big/ratconv.go b/src/math/big/ratconv.go
index 3b43b19f0e..7aed289218 100644
--- a/src/math/big/ratconv.go
+++ b/src/math/big/ratconv.go
@@ -202,6 +202,11 @@ func scanExponent(r io.ByteScanner, binExpOk bool) (exp int64, base int, err err
 
 // String returns a string representation of x in the form "a/b" (even if b == 1).
 func (x *Rat) String() string {
+	return string(x.marshal())
+}
+
+// marshal implements String returning a slice of bytes
+func (x *Rat) marshal() []byte {
 	var buf []byte
 	buf = x.a.Append(buf, 10)
 	buf = append(buf, '/')
@@ -210,7 +215,7 @@ func (x *Rat) String() string {
 	} else {
 		buf = append(buf, '1')
 	}
-	return string(buf)
+	return buf
 }
 
 // RatString returns a string representation of x in the form "a/b" if b != 1,
diff --git a/src/math/big/ratmarsh.go b/src/math/big/ratmarsh.go
index b82e8d4ae8..fbc7b6002d 100644
--- a/src/math/big/ratmarsh.go
+++ b/src/math/big/ratmarsh.go
@@ -59,8 +59,10 @@ func (z *Rat) GobDecode(buf []byte) error {
 
 // MarshalText implements the encoding.TextMarshaler interface.
 func (x *Rat) MarshalText() (text []byte, err error) {
-	// TODO(gri): get rid of the []byte/string conversion
-	return []byte(x.RatString()), nil
+	if x.IsInt() {
+		return x.a.MarshalText()
+	}
+	return x.marshal(), nil
 }
 
 // UnmarshalText implements the encoding.TextUnmarshaler interface.